Phase demodulation method and circuit

ABSTRACT

An electrical circuit and method for de-modulation and carrier recovery of PSK modulated carrier signals in analog domain are described. A portion of the received PSK-modulated carrier signal is passed through a signal multiplication circuit to obtain a frequency-multiplied carrier that is absent of the PSK modulation, which is then passed through a frequency dividing circuit to obtain a reference carrier at the received carrier frequency. The reference signal is then mixed with the received PSK-modulated carrier signal to obtain a de-modulated baseband signal. The method may be used in heterodyne receivers of optical BPSK and QPSK signals.

FIELD OF THE INVENTION

The invention generally relates to methods and systems for detectingsignals modulated using phase shift keying (PSK), and more particularlyrelates to a method and circuit for demodulating PSK-modulated carrierwave signals.

BACKGROUND OF THE INVENTION

The use of phase modulated optical signals in optical communicationsystems offers considerable advantages over on-off keying of opticalsignals intensity in many cases. Examples of phase modulation formatsthat have attracted particular attention in optical applications includevarious phase-shift keying (PSK) modulation formats, such as binary PSK(BPSK) and quadrature PSK (QPSK). However, demodulating such signals ismore complicated than detecting and demodulating intensity modulatedsignals. One known phase demodulation scheme includes the use of delayline interferometers, in which an input PSK modulated optical signal iscoherently combined with a copy thereof that is delayed by one PSKsymbol. One disadvantage of such schemes is that they are not bit rateflexible, i.e. the fixed delay line only decodes signals modulated at acertain bit rate. Furthermore, the delay lines may have to be ratherlong; for example, a 1 Gbit/s signal requires a delay line that is about20 cm long, which is not practical in small form factor components.

Another known PSK decoding schemes uses optical homodyne detection, inwhich light from a local oscillator (LO) laser is mixed with the PSKmodulated light of a transmitter laser. Both the LO laser and thetransmitter laser can be made stable, and the LO laser can be opticallylocked to the transmitter laser. This solution is however not very costefficient.

In a conventional scheme of optical coherent homodyne reception thedetected signals from one or more photodiodes are digitized, and thephase detection is done in a digital signal processor (DSP). However,sufficiently fast digitizers and DSPs may be expensive and power hungry.In some schemes with optical LO phase locking, the control signal of aphase tracking circuit itself can be used as a demodulated signal. Thishowever works only for limited data rates.

In the case of coherent heterodyne reception, e.g., when wavelengths ofthe LO and transmitter lasers differ, the tracking of the phase can bedone in the electric domain instead of the optical domain. Knowntechniques however typically require a fast tunable voltage controlledoscillator (VCO), which may be expensive; furthermore, designing asufficiently robust control loop using a VCO is a non-trivial task.

Accordingly, it may be understood that there may be significant problemsand shortcomings associated with current solutions and technologies fordemodulating PSK modulated electrical and optical signals, includingthose used in high-speed optical communication systems.

SUMMARY OF THE INVENTION

Accordingly, an aspect of the present disclosure relates to a method andapparatus for demodulating an optical PSK signal wherein the optical PSKsignal is first converted into an electrical carrier that isPSK-modulated, and then the PSK-modulated electrical carrier isde-modulated using an analog electrical circuit. Another aspect of thepresent disclosure relates to a method and circuit for demodulatinganalog electrical signals comprising a PSK-modulated carrier wave.

An aspect of the present disclosure relates to a method of demodulatinga received carrier signal that is modulated using a phase shift keying(PSK) modulation format, the method comprising: a) splitting thereceived carrier signal into two analog PSK-modulated signals, eachcomprising a PSK-modulated carrier wave signal with a carrier wavefrequency f that may vary in time; b) passing a first of the two analogPSK-modulated signals through a multiplying circuit to obtain afrequency-multiplied carrier signal; c) passing the frequency multipliedcarrier signal through a first frequency dividing circuit to obtain afirst reference carrier wave signal with the carrier wave frequency f,and, d) mixing the first reference carrier wave signal with the secondof the two analog PSK-modulated signals using a first RF signal mixer toobtain a first de-modulated signal therefrom.

In accordance with an aspect of the present disclosure, step (b) of themethod may include directing a first of the two analog PSK-modulatedsignals into two input ports of an electrical signal mixer. Inaccordance with one aspect of the disclosure, step (c) of the method mayinclude passing the frequency-multiplied carrier signal through anelectrical switching circuit that is configured to switch a transmissionstate thereof in phase with every peak of an input signal. Theelectrical switching circuit may be an ON-OFF switching circuit, and maycomprise an electrical frequency-dividing flip-flop circuit.

In accordance with an aspect of the present disclosure, the receivedcarrier signal may comprise a binary PSK (BPSK) modulated signal, andstep (b) may include passing the first of the two analog PSK-modulatedsignals through a signal squaring circuit to obtain a frequency-doubledsignal.

In accordance with an aspect of the present disclosure, the receivedcarrier signal may comprise a quadrature PSK (QPSK) modulated signal,and step (b) may include passing the first of the two analogPSK-modulated signals through two signal squaring circuits in series.

In accordance with an aspect of the present disclosure, the receivedcarrier signal may comprise a 2^(M)PSK modulated signal, where M is aninteger greater than 0, and step (b) may include passing the first ofthe two analog PSK-modulated signals through M signal squaring circuitsconnected in series.

In accordance with an aspect of the present disclosure, the receivedcarrier signal may comprise a 2^(M)PSK modulated signal, where M is aninteger greater than 0, and step (c) may include passing thefrequency-multiplied carrier wave signal through M frequency-dividingelectrical flip-flop circuits connected in series.

One aspect of the present disclosure provides an electrical circuit fordemodulating a received carrier signal that is modulated using a PSKmodulation format, the electrical circuit comprising: an input signalsplitter configured to split the received carrier signal into two analogPSK-modulated signals, each comprising a PSK-modulated carrier wave witha carrier wave frequency f that may vary in time; a multiplying circuitdisposed to receive a first of the two analog PSK-modulated signals andconfigured to convert it into a frequency-multiplied carrier signal; afrequency dividing circuit configured to convert the frequencymultiplied carrier signal into a first reference carrier wave signalwith the carrier wave frequency f, a first electrical signal mixerconfigured to mix the first reference carrier wave signal with thesecond of the two analog PSK-modulated signals to extract a de-modulatedsignal therefrom; and, an electrical transmission line connecting theinput electrical signal splitter with the first RF signal mixer. Thefrequency-multiplying circuit may comprise one or more electrical signalmixers connected in series. The first frequency dividing circuit maycomprise one or more electrical flip-flop circuits connected in seriesso as to down-covert the frequency-multiplied analog carrier signal tothe carrier wave frequency f.

An aspect of the present disclosure provides an electrical circuit fordemodulating a received carrier signal that is modulated using a2^(M)PSK modulation format where M is an integer greater than 0, theelectrical circuit comprising: an input signal splitter configured tosplit the received carrier signal into two analog 2^(M)PSK-modulatedsignals, each comprising a 2^(M)PSK-modulated carrier wave with acarrier wave frequency f that may vary in time; a multiplying circuitdisposed to receive a first of the two analog PSK-modulated signals andconfigured to convert it into a frequency-multiplied analog carriersignal with a frequency 2^(M)f, the multiplying circuit comprising Msignal squaring circuits connected in series; a frequency dividingcircuit configured to convert the frequency multiplied analog carriersignal into a first analog reference carrier wave signal with thecarrier wave frequency f, the frequency dividing circuit comprising Mfrequency-dividing flip-flop circuits; a first electrical signal mixerconfigured to mix the first analog reference carrier wave signal withthe second of the two analog 2^(M)PSK-modulated signals to extract ade-modulated signal therefrom; and, an electrical transmission lineconnecting the input electrical signal splitter with the firstelectrical signal mixer.

An aspect of the present disclosure provides a method of demodulating anoptical PSK-modulated signal, the method comprising: a) using an opticalheterodyne receiver to obtain a PSK-modulated electrical carrier signalhaving a carrier frequency f that may vary in time and that is greaterthan a PSK modulation rate R_(mod) of the optical PSK-modulated signal;b) splitting the PSK-modulated electrical carrier signal into two analogPSK-modulated signals, each comprising a PSK-modulated carrier wavesignal with the carrier wave frequency f, c) passing a first of the twoanalog PSK-modulated signals through a multiplying circuit to obtain afrequency-multiplied carrier signal; d) passing the frequency multipliedcarrier signal through a first frequency dividing circuit to obtain afirst reference carrier wave signal with the carrier wave frequency f,and, e) mixing the first reference carrier wave signal with the secondof the two analog PSK-modulated signals using a first RF signal mixer toobtain a first de-modulated signal therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings, which may be not to scale and inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a schematic block diagram of an electrical circuit fordemodulating a PSK-modulated carrier wave signal that incorporates amultiplying circuit and a frequency divider for generating a homodynereference signal;

FIG. 2A is a plot illustrating an example BPSK-modulated carrier wavesignal;

FIG. 2B is a plot illustrating the example BPSK-modulated carrier wavesignal of FIG. 2A after the multiplying circuit in FIG. 1;

FIG. 2C is a plot illustrating the example BPSK-modulated carrier wavesignal of FIG. 2A after the frequency divider in FIG. 1;

FIG. 2D is a plot illustrating an output demodulated BPSK signal afterthe output mixer in FIG. 1;

FIG. 3 is a schematic block diagram illustrating an embodiment of thecircuit of FIG. 1 for demodulating BPSK signals, with an electricalsignal mixer in the multiplying circuit;

FIG. 4A is a schematic diagram illustrating an electrical T flip-flopsuitable for use as a frequency divider by two in the circuits of FIGS.1 and 3;

FIG. 4B is a schematic diagram illustrating an electrical D flip-flopsuitable for use as a frequency divider by two in the circuits of FIGS.1 and 3;

FIG. 5 is a schematic block diagram illustrating an embodiment of thecircuit of FIG. 1 incorporating an edge-sharpening filter;

FIG. 6A is a plot illustrating an electrical signal at the output of thesignal squaring circuit in FIG. 5;

FIG. 6B is a plot illustrating an electrical signal at the output of theedge-sharpening filter in the circuit of FIG. 5;

FIG. 6C is a plot illustrating an electrical signal at the output of thefrequency dividing electrical flip-flop in the circuit of FIG. 5;

FIG. 7 is a schematic block diagram illustrating an embodiment of thecircuit of FIG. 5 incorporating a pass-band filter for suppressingamplitude modulation;

FIG. 8A is a plot illustrating an BPSK-modulated carrier wave signalwith extraneous amplitude modulation at an input of the circuit of FIG.7;

FIG. 8B is a plot illustrating the transformation of the BPSK-modulatedcarrier wave signal of FIG. 8A after the squaring circuit in the circuitof FIG. 7;

FIG. 8C is a plot illustrating the transformation of the BPSK-modulatedcarrier wave signal of FIG. 8A after the first band pass filter in thecircuit of FIG. 7;

FIG. 8D is a plot illustrating the transformation of the BPSK-modulatedcarrier wave signal of FIG. 8A after the frequency divider in thecircuit of FIG. 7;

FIG. 8E is a plot illustrating the transformation of the BPSK-modulatedcarrier wave signal of FIG. 8A after the second band pass filter in thecircuit of FIG. 7;

FIG. 8F is a plot illustrating the de-modulated BPSK signal at theoutput of the circuit of FIG. 7;

FIG. 8G is a plot illustrating an eye-diagram of the de-modulated BPSKsignal at the output of the circuit of FIG. 7;

FIG. 9 is a schematic block diagram illustrating an embodiment of thecircuit of FIG. 5 for demodulating BPSK signals incorporating twoparallel frequency dividers for creating two homodyne references with aphase-shift therebetween;

FIG. 10 is a schematic block diagram illustrating an embodiment of thecircuit of FIG. 9 without intermediate filters;

FIG. 11 is a schematic block diagram of a circuit for demodulating QPSKmodulated carrier wave signals;

FIG. 12 is a schematic block diagram of an optical heterodyne receiverof optical PSK signals incorporating an electrical circuit fordemodulating PSK-modulated carrier signals;

FIG. 13 is a flowchart of a method for demodulating a PSK-modulatedcarrier wave signal in analog signal domain;

FIG. 14 is a schematic block diagram illustrating an embodiment of theoptical heterodyne receiver of FIG. 11.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticalcircuits, circuit components, techniques, etc. in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the descriptionof the present invention. All statements herein reciting principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Furthermore, the following abbreviations and acronyms may be used in thepresent document:

PSK Phase Shift Keying

BPSK Binary Phase Shift Keying

QPSK Quaternary Phase Shift Keying

QAM Quadrature Amplitude Modulation

RF Radio Frequency

CMOS Complementary Metal-Oxide-Semiconductor

GaAs Gallium Arsenide

InP Indium Phosphide

LiNO₃ Lithium Niobate

PIC Photonic Integrated Circuits

SOI Silicon on Insulator

SiGe Silicon Germanium

Note that as used herein, the terms “first,” “second” and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The word‘using,’ when used in a description of a method or process performed bya device, component, or circuit, is to be understood as referring to anaction performed by device, component, or circuit itself or by anelement thereof rather than by an external agent. The term ‘analog’refers to signals that encode information in a continuously varyingparameter or parameters, such as for example electrical field, voltage,or current, and to circuits configured to respond to the continuouslyvarying parameter or parameters to process that information; the term‘analog’ is used herein to distinguish from digital signals or circuitsthat encode or process information by switching between a finite set ofvalues or states. The terms ‘carrier’ and ‘carrier wave’ are used hereininterchangeably to refer to a periodic or quasi-periodic signal, such asa sinusoidal wave, that may be modulated in phase, frequency, oramplitude and that is characterized by a carrier wave frequency that istypically greater than the modulation rate. In the context of thepresent disclosure, “RF” may refer to frequencies ranging from a fewkilohertz (kHz) to tens of gigahertz (GHz).

With reference to FIG. 1, there is schematically illustrated an exampleelectrical circuit 100 for demodulating a received carrier signal 101that is modulated using a phase shift keying (PSK) modulation format.The received carrier signal 101 may be an electrical RF signal and mayalso be referred to as the received PSK signal, and may be in the formof an electrical carrier wave X(t) having a carrier wave frequency f anda PSK modulated phase ϕ(t), X(t)=A·exp[j2πft+jϕ(t)], whereϕ(t)=ϕ_(mod)(t)+ϕ₀(t), with ϕ_(mod)(t) denoting the PSK modulationcomponent and ϕ₀(t) denoting a background component of the carrier wavephase; ϕ₀(t) may slowly vary in time, at a rate that is typicallysmaller than the modulation rate R_(mod) of the PSK modulation, due to,for example, noise or circuit non-idealities, effectively resulting inslight variations in the apparent carrier wave frequency of the receivedPSK signal 101.

Generally the PSK modulation may switch the PSK phase componentϕ_(mod)(t) between K possible phase values, K=2, 3, . . . , separated by2π/K radian (rad); PSK modulation of this type may be referred to asK-PSK. In representative embodiments K=2^(M), and the PSK modulation mayswitch the phase ϕ_(mod)(t) between 2^(M) possible phase values, M=1, 2,. . . , separated by 2π/2^(M) radian (rad); PSK modulation of this typemay be referred to as 2^(M)PSK, with M=1 known as binary PSK (BPSK), andM=2 known as quaternary PSK or quadrature PSK (QPSK). Exampleembodiments described hereinbelow relate to demodulating of BPSK andQPSK modulated carrier wave signals. It will be appreciated however thatprinciples described hereinbelow may be extended to higher-order PSKsignals as well, including 2^(M)PSK with M>2, and generally to K-PSKwith K≥2.

The carrier wave frequency f and/or the background phase ϕ₀ of the inputPSK signal 101 may vary in time, for example due to noise intransmission or in preceding signal-processing circuits. The PSKdemodulating circuit 100 is configured to generate, using mostly analogelectrical circuitry, a first reference carrier wave signal 121 thataccounts for these variations and may be used as a homodyne reference inthe demodulation process. The PSK demodulating circuit 100 may includean input signal splitter 105 that is configured to split the inputanalog PSK signal 101 into two analog PSK signals 102 and 103 and todirect one of them along a carrier recovery path 151 and another—along asignal path 152, to be re-combined at a first output electrical signalmixer 130. Each of the analog PSK signals 102 and 103 is in the form ofa PSK-modulated carrier wave with the carrier wave frequency f and phaseϕ that vary in time in substantially the same way in each of the twosignals 102 and 103.

The signal path 152 may be in the form of an electrical transmissionline 103 that connects one of the output ports of the input signalsplitter 105 to one of the input ports of the first output signal mixer130. In some embodiments, it may include other circuit elements, devicesor sub-circuits such as, for example, a signal amplifier. In oneembodiment the carrier recovery path 151 has an electrical length thatmatches the electrical length of the signal path 152 so thatsubstantially no or little phase difference is generated for signalspropagating along these two paths from the input splitter 105 to thefirst output mixer 130. In one embodiment one of these paths 152, 152may include a π/2 phase shifter, or a tunable phase shifter that may betuned to provide a desired electrical phase difference between the paths151 and 152. The issue of electrical lengths of the paths 151 and 152may be of a small concern at carrier frequency f that is on the order orsmaller than about 10-20 GHz provided that the circuit 100 issufficiently compact, so both of the paths 151 and 152 are shortcompared to the carrier wavelength v/f, where v is the effective signalpropagation speed in the circuit.

The carrier recovery path 151 is configured to convert the first analogPSK-modulated carrier signal 102 into an analog reference carrier wavesignal 121 that is substantially absent of the PSK modulation butretains variations in the carrier frequency f and/or background phase ϕ₀that is present in the received PSK signal 101. For that purpose, thefirst analog PSK-modulated signal 102 is first directed to pass througha multiplying circuit 110 that is configured to convert it into afrequency-doubled, or generally frequency-multiplied analog carriersignal 111 with a frequency that is multiple off but that issubstantially absent of the PSK modulation that was present in thereceived PSK modulated signal 101. The frequency-multiplied analogcarrier signal 111 is then passed through a first frequency dividingcircuit 120 that is configured to convert the frequency-multipliedanalog carrier signal 111 into the reference carrier wave signal 121with the carrier wave frequency f. In an embodiment wherein the receivedcarrier is K-PSK modulated, the multiplying circuit 110, which may alsobe referred to herein as the signal multiplying circuit 110, may beconfigured to multiply the PSK modulated analog carrier wave signal 102X(t) by itself K times, and to output the frequency-multiplied signal111 that is proportional to [X(t)]^(K) and has a multiplied carrierfrequency Kf. The frequency dividing circuit 120 may be then a circuitthat divides the frequency of its received signal 111 by K, therebyrecovering the carrier frequency of the received carrier signal 101. Thereference carrier wave signal 121, which may also be referred to hereinas the first (analog) reference carrier wave signal 121, or simply asthe first reference signal 121, is then mixed with the second PSKmodulated signal 103 in the first output signal mixer 130, producing anoutput signal 131 that may be in the form of, or include, a de-modulatedsignal carrying the PSK modulation information in the baseband. Thefirst output signal mixer 130 may be embodied using any suitablenon-linear electrical circuit or device that outputs one or moreproducts of two input signals, including a signal at a frequency that isequal to the difference between frequencies of the input signals. Suchcircuits are well-known in the art as frequency mixers and may beembodied using one or more diodes or transistors. Since the two signals121 and 103 that are input to the first output signal mixer 130 are ofthe same carrier wave frequency f the de-modulated signal 131 is abaseband signal generally proportional to sin(ϕ_(m)(t)+ϕ₁), where ϕ₁ isa constant or slowly varying phase offset. Here, “slowly” means at arate that is smaller than the PSK modulation rate R_(mod), and typicallyat a rate that is at least 10 times smaller than the carrier frequencyf. The input signal splitter 105 may be in the form of a simple passivesignal splitter, or may be a component or circuit that generates orproduces the first and/or second PSK signals 102, 103 that substantiallycopy the time dependence of the carrier wave frequency f and phase ϕ(t)of the input PSK signal 101.

With reference to FIGS. 2A-2D, in one embodiment the receivedPSK-modulated signal 101 is in the form of a BPSK modulated carrierwave, which may be mathematically represented by a sine wave with thecarrier wave frequency f and the phase ϕ(t) that undergoes a sequence ofjumps by π rad, as illustrated in FIG. 2A. The carrier wave frequency fmay slightly vary with time due to noise or circuit non-idealities. Themultiplying circuit 110 in this embodiment may be a squaring circuitthat outputs substantially a square X²(t) of its input PSK-modulatedsignal X(t) 102. In this embodiment, the output frequency-multipliedsignal 111, that may be denoted Y(t), is a frequency-doubled signaloscillating at a frequency 2f that is double the carrier wave frequencyf of the input PSK modulated signal 101. The π rad jumps in phase of thefrequency-doubled signals 111 are substantially eliminated by thesquaring operation in the signal squaring circuit 110, as illustrated inFIG. 2B. Advantageously, the frequency-doubled signal 111 retains theslower, non-modulation-related variations in frequency f and/or phase ϕthat may be present in the input PSK modulated signal 101. The frequencydividing circuit 120 then performs a frequency-division-by-2 operationon the frequency-doubled signal 111 and outputs the reference carriersignal 121 at the carrier wave frequency f which is illustrated in FIG.2C and which also retains the non-BPSK variations in frequency f and/orphase ϕ of the input PSK modulated signal 101. The first output signalmixer 130, which is configured to mix the reference carrier wave signal121 with the second BPSK-modulated carrier wave signal 103, outputs thede-modulated signal 131 which in this embodiment is a binary signal asschematically illustrated in FIG. 2D.

Turning now to FIG. 3, in one embodiment 100 a the signal multiplyingcircuit 110 is a signal-squaring circuit that may be in the form, orinclude, an electrical signal mixer 116 that has two input ports and oneoutput port and that is configured to mix the first analog PSK-modulatedcarrier signal 102 with a copy or a portion of itself 102 a, thusproducing at its output port a signal Y(t) proportional to the productX(t)·X(t) of the input PSK-modulated carrier signal X(t) 101 or 102. Theelectrical signal mixer 116 may also be referred to herein as the input(electrical) signal mixer 116. The signal squaring circuit 110 may alsoinclude at its input a second signal splitter 114 that splits the firstPSK signal 102 into two portions or copies 102 a and 102 b, which arethen directed to the input ports of the second signal splitter 114.

The frequency dividing circuit 120 may be implemented as, for example, aswitching circuit 220 that is configured to block every other peak oroscillation of its input signal, or to switch a transmission state or anoutput state thereof in phase with every peak or oscillation of itsinput signal, such as for example a binary counter that generates oneoutput pulse for every two input pulses or oscillations. In oneembodiment, the signal switching circuit 220 may be for example in theform of a frequency dividing electrical flip-flop circuit, which iswell-known in the art. FIGS. 4A and 4B illustrate the frequency-dividingswitching circuit 220 embodied as a T (toggle) flip-flop and a D (delay)flip-flop, respectively, each of which may be used as thefrequency-dividing circuit 120. A suitable flip-flop circuit may beimplemented for example using pairs of bipolar junction or field effecttransistors as also well known in the art. For example when the ‘T’input in the T flip-flop is held ‘high,’ the T flip-flop divides the‘clock’ frequency by two. By way of example, the carrier wave frequencyf may be an RF frequency, for example 12 GHz. Then the flip-flop circuit220 that receives at its clock input the frequency-doubled signal 111 at24 GHz, will output a signal at the carrier wave frequency of 12 GHz.

An electrical flip-flop circuit such as illustrated in FIGS. 4A and 4Bmay be configured to be triggered by a rising or falling ‘edge’ of aninput pulse signal, which in the embodiment 100 a of FIG. 3 isexemplified by the frequency-doubled signal 111. However, a sine wavedoesn't have well-defined ‘edges,’ which may potentially lead to ajitter at the output of the flip-flop 220. Therefore in some embodimentsit may be desirable to add an edge-sharpening filter prior to thefrequency dividing flip-flop.

With reference to FIG. 5, there is illustrated an embodiment 100 b ofthe PSK demodulating circuit 100 which incorporates an edge-sharpeningfilter 215 between the signal squaring circuit 110 and the frequencydividing flip-flop 220. The edge-sharpening filter 215 may be configuredto implement, substantially, an approximation to the signum functionsign(Y(t)). For example it may be configured to output a ‘high’ valuewhen the input frequency-doubled or, generally, frequency-multipliedsignal 111 Y(t) is positive, and a ‘low’ value, for example zero, whenthe input frequency-doubled or, generally, frequency-multiplied signal111 Y(t) is negative, or vice versa. Such a filter may be implemented,for example, using a high-gain limiting amplifier that is configured tolimit its output to a constant ‘high’ or ‘low’ value whenever the inputsignal 111 Y(t) rises in value above a small fraction of its amplitude;other embodiments may be possible as will be apparent to those skilledin the art.

Referring also to FIGS. 6A-6C while continuing to refer to FIG. 5, theedge-sharpening filter 215 converts the frequency-doubled, or generallyfrequency-multiplied, signal 111, which is schematically illustrated inFIG. 6A, into a frequency-doubled square-wave signal 111 a that isschematically illustrated in FIG. 6B. The frequency dividing flip-flop220 then down-converts the frequency-doubled square-wave signal 111 ainto a square-wave signal at the carrier frequency f that is half of thedoubled frequency 2f of its input signal 111 a, as schematicallyillustrated in FIG. 6C. An optional passband or low-pass filter 225 maybe added after the frequency-dividing circuit 220 so as to filter outall higher-order harmonics nf, n=2, of the input carrier frequency fwhile allowing the carrier frequency f to propagate, resulting in agenerally sinusoidal reference carrier wave signal 121 at frequency f asillustrated in FIG. 2C. In one embodiment, an output low-pass filter(LPF) 235 may further be added after the first output signal mixer 130so as to eliminate frequency-doubled components that may appear at theoutput of the mixer 130. LPF 235 may have a cutoff frequency of theorder of the BPSK modulation rate R_(mod) or slightly lower.

We found that the PSK demodulation technique described hereinabove withreference to FIGS. 1-6 may also be applied when the received PSK signal101 includes a degree of amplitude modulation. An extraneous amplitudemodulation may appear, for example, when the input PSK modulated carrierwave 101 was produced by an optical heterodyne receiver of an opticalBPSK signal that was in turn produced using an optical phase modulatorbased on a semiconductor optical amplifier (SOA). Optical BPSK signalsproduced by Mach-Zehnder Modulators (MZM) driven with a ±V_(π) drivevoltage may also have a significant amplitude modulation component, asthe optical amplitude of MZM modulated light may undergo a zero crossingbetween subsequent changing symbols. We found, however, that undesirableeffects of such amplitude modulation may be mitigated when using themethod of the present disclosure, for example by adding a band-passfilter (BPF) after the signal squaring circuit.

Referring to FIG. 7, there is illustrated a variation of the PSKdemodulating circuit of FIG. 5 that is indicated here as PSKdemodulating circuit 100 c. In the shown embodiment the PSK demodulatingcircuit 100 c is generally as illustrated in FIG. 5, but furtherincludes a BPF 212 that is inserted in the carrier recovery path 151 ofthe circuit between the multiplying or squaring circuit 110 and thefrequency dividing circuit 120. The edge-sharpening filter 215 may beomitted in some embodiments. The pass band of the BPF 212 is centered atthe carrier-multiplied frequency of the signal 111 at the output of thesignal multiplying circuit 110, which is for example 2f if circuit 110is a signal squaring circuit, and is configured so as to remove orsuppress spectral components of the frequency-multiplied signal 111which are due to the extraneous or residual amplitude modulation of thereceived PSK signal 101.

With reference to FIGS. 8A-8G, there is illustrated the transformationof an input BPSK modulated signal 101 as it propagates along the carrierrecovery path 151 of the PSK de-modulating circuit 100 c in anembodiment wherein the multiplying circuit 110 is a signal squaringcircuit and the frequency dividing circuit 120 is a frequency-dividingflip-flop. All plots in these figures are generated using a measuredBPSK modulated carrier signal with the carrier frequency f of 12 GHzfrom an optical heterodyne receiver for an example model circuit 100 c.The incoming BPSK modulated carrier wave signal 101 has residualamplitude modulation, as illustrated in FIG. 8A. This signal is splitinto the two signal parts 102 and 103 and the first signal part 102 isused to generate the reference carrier wave signal 121, against whichthe second signal part 103 of the input BPSK signal is de-modulatedusing the first output signal mixer 130. The squared signal 111, asshown in FIG. 8B, is free of any phase jumps but shows significantamplitude variations. The BPF 212 removes most of these amplitudevariations, and the resulting signal at the output of the BPF 212 isshown in FIG. 8C. The edge-sharpening filter 215 in this embodimentgenerates a positive fixed voltage at its output while its input ispositive, and a low fixed voltage while its input voltage is negative.The frequency of the resulting rectangular signal is then divided by twoby the frequency divider 120, with the resulting square-wave signal withperiod 1/f shown in FIG. 8D. The filter 225 with a pass-band centered atthe carrier wave frequency f removes the harmonic components of thesquare wave, resulting in a generally sinusoidal reference wave 121shown in FIG. 8E. This reference carrier wave is then used in the mixerstage 130 to de-modulate the incoming BPSK modulation, converting it tothe baseband. The optional LPF 235 may be used to remove high frequencycomponents of the output signal from the mixer 130 when desired, and thedecoded signal is shown in FIG. 8F. The resulting eye diagramconstructed for the de-modulated signal is well open, as shown in FIG.8G.

Turning now to FIG. 9, there is illustrated a PSK demodulating circuit100 d in accordance with an embodiment of the present disclosure. Thisembodiment differs from that of FIG. 7 in that it includes a secondfrequency dividing circuit 123 that is connected in parallel with thefirst frequency dividing circuit 120 and is followed by a second outputsignal mixer 133. The carrier recovery path 151 is split into two usinga signal splitter 115, so that a portion of the frequency multipliedanalog carrier signal 111 is passed through the second frequencydividing circuit 123 in a second carrier recovery path 151 a to obtain asecond analog reference carrier wave signal 121 a, which may bephase-shifted relative to the first analog reference carrier wave 121.This second analog reference carrier wave signal 121 a is then mixed,using the second electrical signal mixer 133, with a portion of thesecond analog PSK-modulated signal 103 that propagated along the signalpath 152. Mixed signals from the outputs of the first and second signalmixers 130, 133 are then combined by a signal combiner 245 to obtain abaseband demodulated signal 231. Optionally, the mixed signals from themixers 130, 132 may be filtered using LPF filter or filters 235, whichmay be disposed electrically before or after the signal combiner 245. Inone embodiment this phase shift between the first and second referencecarrier wave signals 121 and 121 a may be substantially 90° at therespective output mixers.

Thus, in the scheme of FIG. 9 two reference signals 121 and 121 a with aphase-shift of, for example, 90° therebetween are produced from the sameinput PSK modulated carrier wave signal 101 using thefrequency-multiplication followed by frequency-dividing technique asgenerally described hereinabove. These two phase-shifted referencesignals 121 and 121 a are used to produce two de-modulated signals 131,132 having a relative phase shift therebetween, which are added togetherat the output of the circuit 100 d. Summing two de-modulated signalsproduced by mixing a PSK-modulated carrier signal with referencecarriers having a relative phase shift therebetween may improve thesignal to noise ratio of the summary signal 231, in particular when thePSK modulation rate R_(mod) is close to the carrier frequency f. In theillustrated embodiment the signal squaring circuit 110 produces thefrequency-doubled signal 111, which is then optionally filtered with theedge-sharpening filter 215 and split into two frequency-doubled signalsby a second signal splitter 115, with one of them directed to the firstfrequency dividing circuit 120 that is followed by the first signalmixer 235 with an optional first narrowband filter 225 therebetween, andthe other directed to pass through the second frequency-dividing circuit123 that is followed by the second signal mixer 235 with an optionalsecond narrowband filter 225 therebetween.

In one embodiment the second frequency-dividing circuit 123 may beconfigured to output the second reference signal 121 a, which isdown-converted to the carrier wave frequency f, that is shifted in phasewith respect to a first reference signal produced by the first frequencydividing circuit 120. The phase shift between the first and secondreference signals 121, 121 a may be, for example, 90°, or π/2 rad. Thefirst and second frequency dividing circuits 120 and 123 may be embodiedas frequency dividing electrical flip-flop circuits that are triggeredat different edges of their respective input signals. For example, thefirst frequency dividing circuit 120 may be in the form of an electricflip-flop that is configured to be triggered by the leading edges of theinput carrier-multiplied signal 111, while the second frequency dividingcircuit 123 may be in the form of an electric flip-flop that isconfigured to be triggered by the trailing edges of the inputcarrier-multiplied signal 111, resulting thereby in the reference signal121 a that is delayed in phase by π/2 rad with respect to the firstreference signal 121. In another embodiment, both the first and secondfrequency-dividing circuits 120, 123 may be in the form of, or include,electric flip-flops that are configured to be triggered from the sameedges of the incoming signals, or at crossings of a pre-determinedthreshold signal level, and one of the first and secondfrequency-dividing circuits 120, 123 may additionally include a π/2phase shifter (not shown).

Referring to FIG. 10, there is illustrated an embodiment of the PSKdemodulation circuit of FIG. 9 for use at relatively high carrier wavefrequencies f for example about or beyond 5-10 GHz, where naturalbandwidth limitations of circuit components and/or electricaltransmission lines, e.g., PCB traces in embodiments where the circuit isimplemented in a printed circuit board (PCB), may make one or more ofthe frequency filters 215, 225, and 235 superfluous thereby allowingtheir removal from the circuit. FIG. 10 illustrates an embodiment of thePSK demodulation circuit of FIG. 9 without any of these filters, andwith the squaring circuit 110 embodied using an electrical signalsplitter 114 and an electrical signal mixer 116 as described hereinabovewith reference to FIG. 3.

With reference to FIG. 14, the second reference carrier wave signal 121a may also be obtained from the first reference carrier wave signal 121without adding the second frequency divider 123, as generallyillustrated in the figure. In this embodiment a portion or copy of thefirst reference carrier wave signal 121 may be split off, for exampleusing a signal splitter 117, and passed to a phase shifting device orcircuit 177 that shifts, for example delays, the phase of the portion ofthe first reference carrier wave signal 121 it receives by 90°. It willbe appreciated that the phase shifting device or circuit 177 may beimplemented in a variety of ways, with the choices that may vary independence on the carrier wave frequency f. For example, in oneembodiment the phase shifting device or circuit 177 may be implementedusing an AC-coupled amplifier. This option may be easier to implement atrelatively low carrier wave frequencies, for example in the low-GHzrange and below. In another example, the phase shifting device orcircuit 177 may be implemented using a suitable delay line; this optionmay be more suitable at the carrier wave frequency f in the 10-20 GHzrange.

Although certain details of the technique and circuit of the presentdisclosure for demodulating PSK modulated analog carrier wave signalshave been described hereinabove mainly with reference to demodulation ofBSPK-modulated analog carriers, it will be appreciated that mainprinciples of the technique can be applied to de-modulating higher-orderPSK formats applied to electrical carrier waves, including but notlimited to K-PSK with K>2 and 2^(M)PSK with M>1. Referring back to thePSK demodulation circuit 100 illustrated in FIG. 1, it will beappreciated that if the input signal 101 is a 2^(M)PSK modulated analogcarrier wave at frequency f circuit 100 will de-modulated it if themultiplying circuit 110 is configured to perform M successive squaringoperations, or a single operation of taking the input signal 101 to the2^(M) power, so as to output a frequency-multiplied signal 111 with afrequency up-converted to 2^(M)·f, and the frequency dividing circuit120 is configured to down-convert the frequency-multiplied signal 111 itreceives to the input carrier wave frequency f. Bandwidth limitations ofthe electrical circuit elements used may however put a limit to the PSKmodulation level M that may be de-modulated for a given input carrierwave frequency f.

Referring now to FIG. 11, there is illustrated a PSK demodulationcircuit 200 that is configured to demodulate a QPSK modulated carrierwave signal 201. As illustrated, the QPSK demodulating circuit 200 issimilar to the BPSK de-modulating circuit of FIG. 9, except that circuit200 lacks an output signal combiner 245 and thus outputs two demodulatedsignals 131 and 132, and additionally includes a second signal squaringcircuit 210 a that is disposed in series with the first signal squaringcircuit 210. The input signal splitter 105 splits the received QPSKmodulated carrier signal 201 with the carrier wave frequency f into twoQPSK modulated carrier wave signals 202 and 203, and directs them alongthe carrier recover path 151 and the signal path 152, respectively. Eachof the two QPSK modulated carrier wave signals 202 and 203 has thecarrier wave frequency f and retains the time dependence characteristicsof the received QPSK carrier signal 201. The second QPSK modulatedcarrier wave signal 203 is provided to one of the input ports of each ofthe two output signal mixers 130 and 133. The two serially connectedsquaring circuits 210 and 210 a are followed by two frequency dividingcircuits in each of the two carrier recovery paths 151 and 151 a. Thefirst frequency dividing circuit 220 may be shared between the twocarrier recovery paths. The first carrier recovery path 151 includes asecond frequency dividing circuit 222 connected in series with the firstfrequency dividing circuit 220 and outputs the first reference carrierwave 121. A third frequency dividing circuit 223 is connected inparallel with the second frequency dividing circuit 222 by means of asignal splitter 115. The second output electrical signal mixer 133 isconnected to an output of the third frequency dividing circuit 223 foroutputting a second de-modulated signal 132. Each of the frequencydividing circuits 220, 222 and 223 is configured to divide the frequencyof their respective input signals by two, and may be embodied, forexample, as a frequency-dividing flip-flop or switch as describedhereinabove.

The two signal squaring circuits 210, 210 a disposed in sequence embodya multiplying circuit that brings the first QPSK-modulated analogcarrier wave signal 202 to the fourth power, thereby up-converting it infrequency to 4f and removing π/2 phase shifts originating from the QPSKmodulation of the received carrier signal 201. The resultingfrequency-multiplied signal 211 may generally be in the form of asinusoid with a nominal frequency 4f, which may however slowly vary infrequency, phase and amplitude due to noise and/or non-idealities inpreceding circuits, as inherited from the corresponding variations inthe received QPSK-modulated carrier wave signal 201. Here, “slowly”means at a rate that is much smaller than the corresponding carrierfrequency f and smaller than the QPSK modulation rate R_(mod), forexample at a rate that is less than f/10 and typically not greater thanf/100. The frequency-multiplied, or in this case frequency-quadrupled,analog carrier signal 211 may be optionally passed through anedge-sharpening filter 215, and then directed to the firstfrequency-dividing circuit 220. An additional passband filter centeredat the quadrupled frequency 4f may be disposed prior to thefrequency-dividing circuit 122 to filter out residual amplitudemodulation if desired, as described hereinabove with reference to FIG.7. The frequency-dividing circuit 120 divides the frequency of thereceived signal 211 by two, outputting a frequency-doubled carriersignal 213, which is then processed similarly to the frequency-doubledsignal 111 in circuit 110 d of FIG. 9 to obtain first and secondreference signals 121 and 121 a using two frequency-dividing circuits222, 223. The third frequency-dividing circuit 223 is configured to addor subtract a 90° phase shift to its output reference carrier signal 121a relative to the output signal 121 of the second frequency-dividingcircuit 222, as described hereinabove with reference to FIG. 9 and thefrequency dividing circuits 120 and 123. Signal mixers 130 and 133 mixthe first and second reference signals 121 and 121 a having a 90°relative phase shift therebetween with split-off portions of the secondQPSK modulated carrier wave signal 203 to obtain the first and secondbaseband de-modulated signals 131 and 132 that represent an ‘in-phase’(I) and ‘quadrature’ (Q) modulation components of a QPSK modulationsignal. The 90° phase difference in relative phases of the mixingsignals in the signal mixers 130 and 133 that is desired to obtain thequadrature modulation signals 131 and 132 may be realized by using thefrequency dividers 120 and 123 triggered by differently inclined edgesof the frequency-doubled signal 213, so that one of them is triggered bythe rising edges of the received signal and the other—by the fallingedges of the received signal. In another embodiment, a π/2 phaseshifter, such as for example a 1/(4f) delay line, may be used to providethe desired phase shift between the first and second reference signals121 and 121 a at the respective mixers. In another embodiment, thedesired π/2 phase shift may be added to one of the split-off QPSKsignals 203 prior to mixing with one of the corresponding referencesignals 121 and 121 a. Filters 215, 225, and 235 may be substantially asdescribed hereinabove with reference to FIGS. 5, 7, and 9; one or moreof these filters may be omitted in some embodiments.

In another embodiment of the QPSK demodulation circuit 200, the secondreference carrier wave signal 121 a may be obtained from the firstreference carrier wave signal 121 without adding the third frequencydivider 223 and using a phase shifting device and circuit 117, asgenerally illustrated in FIG. 14 and described hereinabove. In thisembodiment the multiplier 110 in FIG. 14 may be in the form of the twosquaring circuits 210 and 210 a in series as illustrated in FIG. 11, andthe frequency divider 120 of FIG. 14 may be in the form of the first andsecond frequency dividers by two 220, 222 connected in series, asillustrated in FIG. 11. In this embodiment, a portion or copy of thefirst reference carrier wave signal 121 from the output of the secondfrequency divider 222 may be split off, for example using the signalsplitter 117, and passed to a phase shifting device or circuit 177 thatshifts, for example delays, the phase of the portion of the firstreference carrier wave signal 120 it receives by 90°. The phase shiftingdevice or circuit 177 may be implemented as described hereinabove, forexample using an AC-coupled amplifier or a suitable delay line.

Turning now to FIG. 12, an electrical PSK demodulation circuit 400incorporating principles of the present disclosure may be used in aheterodyne receiver of optical BPSK and QPSK signals. Such a receivermay include an optical reference source 14 that is configured forproducing an optical reference signal 5, an optical mixer 12 that isconfigured for mixing an input optical PSK modulated signal 3 with theoptical reference signal 5 and to output a mixed optical signal 7, and aphotodetector (PD) 16 disposed to receive the mixed optical signal 7 forconverting thereof into an electrical PSK-modulated carrier wave signal401, which is then provided to the PSK demodulating electrical circuit400. The optical front-end of the receiver, including the optical mixer12 and the reference optical source 14, may be embodied for example as aphotonic integrated circuit (PIC) formed in or upon a suitablesubstrate. For example, it may be embodied using the Silicon on Isolator(SOI) technology as a SOI PIC chip 50 with an input optical waveguideport 11, which may also incorporate the PD 16. The optical mixer 12 maybe for example in the form of a 2×2 multi-mode interference (MMI)coupler as known in the art, with one of the input ports connected to aninput optical waveguide for receiving the optical PSK signal 3, andanother coupled to the reference LD 14, with the PD 16 optically coupledto one or both of the output ports of the MMI coupler 12. When coupledto both output ports of the MMI coupler 12 as illustrated in FIG. 12, PD16 may be implemented as a differential photodetector, for exampleformed of two differentially connected photodiodes (not shown), each ofwhich coupled to a different output port of the MIMI coupler 12. Thereference optical source 14 may be embodied using a laser diode (LD),which may be wavelength-stabilized at a reference wavelength thatdiffers slightly from the wavelength of the incoming optical signal 3,with a difference in their optical frequency defining the carrier wavefrequency f of the electrical PSK modulated signal 401 at the output ofthe PD 16. The PSK demodulating electrical circuit 400 may be embodiedfor example as described hereinabove with reference to FIGS. 1, 3, 3, 5,7 and 9-11, with an input electrical port coupled to the output of thePD 16 to receive the PSK-modulated electrical carrier signal 401. Insome embodiments the de-modulated PSK signal or signals 431 from theoutput of the PSK demodulating circuit 400 may be converted to a digitaldomain by an ADC 450 and then sent to a processor 455 for furtherprocessing and decoding. The de-modulated PSK signal may be comprised oftwo de-modulated ‘I’ and ‘Q’ signals, as described hereinabove withreference to FIG. 11. The PSK demodulating circuit 400 may beimplemented in a single semiconductor chip, or may be a hybridelectrical circuit comprised of one or more electrical sub-circuits thatmay be mounted on a support base 450, that for example may be in theform of a PCB. In another embodiment, the support base 450 may be asingle semiconductor substrate or wafer, wherein the circuit elements105, 110, and 120 are integrated. For example, the PSK demodulatingcircuit 400 may be in the form of a CMOS chip or a SiGe chip.

Referring now to FIG. 13, embodiments of the PSK demodulating circuitthat are described hereinabove are configured to implement a method ofdemodulating a received PSK-modulated carrier signal, which enables toperform carrier recovery in the domain of analog signals. An embodimentof the method may be generally described as follows. The method maystart with a step or operation 310 wherein the received analogPSK-modulated carrier is split into two analog PSK-modulated signals,each in the form of a PSK-modulated carrier wave signal with a carrierwave frequency f that may vary in time. At step or operation 320, afirst of the two analog PSK-modulated signals is passed through amultiplying circuit to obtain a frequency-multiplied analog carriersignal that is substantially absent of the PSK modulation. At step oroperation 330, the frequency multiplied analog carrier signal is passedthrough a first frequency dividing circuit to obtain a first analogreference carrier wave signal with the carrier wave frequency f.Finally, at step or operation 340 the first analog reference carrierwave signal is mixed with the second of the two analog PSK-modulatedsignals using a first output electrical signal mixer to obtain a firstde-modulated signal in the baseband. The signal splitting operation 310may be in the form of a passive signal splitting wherein the input PSKsignal is split in two PSK signals of lower signal power, or may be inthe form of making a copy or copies of the received PSK signal so as toretain the frequency vs. time and phase vs. time information of thereceived PSK signal.

In one embodiment, step or operation 320 of the method may includedirecting a first of the two analog PSK-modulated signals into two inputports of an electrical signal mixer. In one embodiment step or operation330 may include passing the frequency-multiplied analog carrier signalthrough an electrical switching circuit that is configured to switch atransmission state thereof from an “ON” state to an ‘OFF” state in phasewith every other peak or oscillation of a signal at the input of theswitch. The electrical switching circuit may be an ON-OFF switchingcircuit configured to switch a transmission state of the switchingcircuit from an “ON” state to an ‘OFF” state. In one embodiment, theelectrical switching circuit may be in the form, or include, anelectrical frequency-dividing flip-flop circuit.

In one embodiment the received carrier signal may be in the form of aBPSK modulated signal, and step or operation 320 may include passing thefirst of the two analog PSK-modulated signals through a signal squaringcircuit, which may be in the form or include an electrical signal mixer.In one embodiment the received PSK-modulated carrier signal may be aQPSK modulated signal, and step (b) may include passing the first of thetwo analog PSK-modulated signals through two signal squaring circuits inseries.

In one embodiment the received carrier signal may be a 2^(M)PSKmodulated carrier wave signal, where M is an integer greater than 0, andstep or operation 320 may include passing the first of the two analogPSK-modulated signals through M signal squaring circuits connected inseries. In one embodiment step or operation 330 may include passing thefrequency-multiplied carrier wave signal through M frequency-dividingelectrical flip-flop circuits connected in series.

In one embodiment, the method may include obtaining, from thefrequency-multiplied carrier signal, a second reference carrier wavesignal that is phase-shifted relative to the first reference carrierwave signal, and mixing the second reference carrier wave signal with atleast a portion of the second of the two analog PSK-modulated signalsusing a second electrical signal mixer.

In one embodiment wherein the received carrier signal is QPSK modulated,step or operation 320 of the method may include sequentially passing thefirst of the two analog PSK-modulated signals through two signalsquaring circuits for converting into a frequency-quadrupled carriersignal. Step or operation 330 may then include sequentially passing atleast a portion of the frequency quadrupled carrier signal through thefirst frequency dividing circuit and a second frequency dividingcircuit. The method may further include passing a portion of a signalfrom an output of the first frequency dividing circuit to a thirdfrequency dividing circuit to obtain the second reference carrier wavesignal that is phase-shifted with respect to the first reference carrierwave signal by 90°, and obtaining, from the second electrical signalmixer, a second de-modulated signal in quadrature with the firstde-modulated signal.

Advantageously, the method and circuit for demodulating PSK modulatedcarrier signals, which principles are described hereinabove withreference to specific embodiments, enable to perform analog-domaincarrier recovery and demodulation of PSK modulated carriers usingwell-known electrical circuits and circuit elements, such as signalmixers and flip-flops, and do not require fast signal digitizers ordigital processors that are capable of working at the carrier frequency.The method and circuit may be used, for example but not exclusively, fordemodulating PSK-modulated carrier signals that appear in opticalheterodyne detection with LO lasers that are not frequency-locked to asignal transmitting source.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Indeed, various other embodiments and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other embodiments andmodifications are intended to fall within the scope of the presentdisclosure. For example one or several amplifiers might be placed withinthe carrier recovery path of the PSK de-modulating circuits describedhereinabove in order to compensate for the losses by the filters and/orother elements in the path. The filters may be implemented as passiveLRC based structures or may contain active elements.

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A method of demodulating a received phase shiftkeying (PSK) modulated carrier signal, the method comprising: a)splitting the received PSK modulated carrier signal into two analogPSK-modulated signals, each comprising a PSK-modulated carrier wave witha carrier wave frequency f; b) passing a first of the two analogPSK-modulated signals through a multiplying circuit to obtain afrequency-multiplied carrier signal absent of PSK modulation; c) passingthe frequency multiplied carrier signal through a first frequencydividing circuit to obtain a first reference carrier wave signal withthe carrier wave frequency f; and, d) mixing the first reference carrierwave signal with the second of the two analog PSK-modulated signalsusing a first electrical signal mixer to obtain a first de-modulatedsignal therefrom; wherein the method further includes: obtaining, fromthe frequency-multiplied carrier signal, a second reference carrier wavesignal that is phase-shifted relative to the first reference carrierwave signal; and, mixing the second reference carrier wave signal withat least a portion of the second of the two analog PSK-modulated signalsusing a second electrical signal mixer.
 2. The method of claim 1,wherein b) comprises directing the first of the two analog PSK-modulatedsignals into two input ports of an electrical signal mixer.
 3. Themethod of claim 1, wherein c) comprises passing the frequency-multipliedanalog carrier signal through an electrical switching circuit that isconfigured to switch a state thereof in phase with an input signalreceived by the electrical switching circuit.
 4. The method of claim 1,wherein c) comprises passing the frequency-multiplied carrier signalthrough a frequency-dividing flip-flop circuit.
 5. The method of claim1, wherein the received PSK modulated carrier signal comprises a binaryPSK (BPSK) modulated signal, and wherein (b) comprises converting thefirst of the two analog PSK-modulated signals modulated signal into afrequency-doubled signal.
 6. The method of claim 1, further comprisingat least one of: passing the frequency-multiplied carrier signal throughan edge-sharpening filter, and passing the first reference carrier wavesignal through a bandpass filter with a passband including the carrierwave frequency f.
 7. The method of claim 1, wherein the received PSKmodulated carrier signal is quadrature-PSK (QPSK) modulated, andwherein: (b) comprises sequentially passing the first of the two analogPSK-modulated signals through two signal squaring circuits forconverting into a frequency-quadrupled carrier signal, (c) comprisessequentially passing at least a portion of the frequency quadrupledcarrier signal through the first frequency dividing circuit and a secondfrequency dividing circuit, and wherein the method further comprises:passing a portion of a signal from an output of the first frequencydividing circuit to a third frequency dividing circuit to obtain thesecond reference carrier wave signal that is phase-shifted with respectto the first reference carrier wave signal by 90°, and obtaining, fromthe second electrical signal mixer, a second de-modulated signal inquadrature with the first de-modulated signal.
 8. The method of claim 1wherein the received PSK modulated carrier signal is binary-PSK (BPSK)modulated, the method further comprising: passing a portion of thefrequency multiplied carrier signal through a second frequency dividingcircuit to obtain the second analog reference carrier wave signal thatis phase-shifted relative to the first analog reference carrier wave;and, combining output signals from the first and second electricalsignal mixers.
 9. An electrical circuit for demodulating a received PSKmodulated carrier signal that is modulated with a quadrature PSK (QPSK)modulation format, the electrical circuit comprising: a first signalsplitter configured to split the received PSK modulated carrier signalinto two analog PSK-modulated signals, each comprising a PSK-modulatedcarrier wave with a carrier wave frequency f that may vary in time; amultiplying circuit disposed to receive a first of the two analogPSK-modulated signals and configured to convert it into afrequency-multiplied carrier signal absent of PSK modulation; a firstfrequency dividing circuit configured to convert the frequencymultiplied carrier signal into a first reference carrier wave signalwith the carrier wave frequency f; a first electrical signal mixerconfigured to mix the first reference carrier wave signal with thesecond of the two analog PSK-modulated signals to extract a firstde-modulated signal therefrom; and, an electrical transmission lineconnecting the first signal splitter with the first electrical signalmixer; wherein the multiplying circuit comprises two signal squaringcircuits connected in series, and wherein the electrical circuit furthercomprises: a second frequency dividing circuit connected in series withthe first frequency dividing circuit; a third frequency dividing circuitconnected in parallel with the second frequency dividing circuit andconfigured to output a second reference carrier wave signal that isshifted in phase relative to the first reference carrier wave signal by90°; and, a second electrical signal mixer connected to an output of thethird frequency dividing circuit for outputting a second de-modulatedsignal.
 10. The electrical circuit of claim 9, wherein the multiplyingcircuit comprises an electrical signal mixer.
 11. The electrical circuitof claim 9, wherein the first frequency dividing circuit comprises anelectrical switching circuit configured to switch a state thereof inphase with an input signal received by the electrical switching circuit.12. The electrical circuit of claim 9, wherein the first frequencydividing circuit comprises a frequency-dividing flip-flop circuit. 13.The electrical circuit of claim 12, further comprising anedge-sharpening filter disposed operationally between the multiplyingcircuit and the first frequency dividing circuit.
 14. The electricalcircuit of claim 9, wherein the received PSK modulated carrier signal ismodulated with a binary PSK (BPSK), and wherein the multiplying circuitcomprises a signal squaring circuit.
 15. The electrical circuit of claim9, further comprising a bandpass filter having a passband including thecarrier wave frequency f and disposed operationally between the firstfrequency dividing circuit and the first electrical signal mixer. 16.The electrical circuit of claim 9 that is integrated in a semiconductorchip.
 17. An optical PSK receiver comprising the electrical circuit ofclaim 9, the optical PSK receiver further comprising an opticalreference source configured for producing an optical reference signal,an optical mixer configured for mixing an input optical PSK modulatedsignal with the optical reference signal and to output a mixed opticalsignal, and a photodetector disposed to receive the mixed optical signalfor converting thereof into the received PSK modulated carrier signal.18. An electrical circuit for demodulating a received PSK modulatedcarrier signal, the electrical circuit comprising: a first signalsplitter configured to split the received PSK modulated carrier signalinto two analog PSK-modulated signals, each comprising a PSK-modulatedcarrier wave with a carrier wave frequency f that may vary in time; amultiplying circuit disposed to receive a first of the two analogPSK-modulated signals and configured to convert it into afrequency-multiplied carrier signal absent of PSK modulation; a firstfrequency dividing flip-flop circuit configured to convert the frequencymultiplied carrier signal into a first reference carrier wave signalwith the carrier wave frequency f, a first electrical signal mixerconfigured to mix the first reference carrier wave signal with thesecond of the two analog PSK-modulated signals to extract a firstde-modulated signal therefrom; and, an electrical transmission lineconnecting the first signal splitter with the first electrical signalmixer; wherein the received PSK modulated carrier signal comprises aBPSK modulated signal, wherein the multiplying circuit comprises asignal squaring circuit configured to output a frequency-doubled carrierwave signal, and wherein the electrical circuit further comprises: asecond frequency dividing circuit connected in parallel with the firstfrequency dividing circuit so as to obtain a second reference carrierwave signal that is phase-shifted relative to the first referencecarrier wave, a second electrical signal mixer configured to mix thesecond reference carrier wave signal with a portion of the second of thetwo analog PSK-modulated signals, and an electrical signal combinerdisposed to combine outputs from the first and second electrical signalmixers to produce an output de-modulated signal.